The process air compressor in most operating ammonia plants is normally the first major bottleneck to increase the ammonia production. The process air compressor for typical average size ammonia plant is a multistage centrifugal machine driven by steam turbine using high-pressure superheated steam. It is one of the major consumers of steam in the plant.
To debottleneck the process air compressor in an existing plant, ammonia plant operators have conventionally used a combination of the following measures:                a. Modification of existing compressor rotor and other essential internals of the compressor;        b. Addition of a parallel new compressor with a driver        c. Increased suction chilling of process air using an expanded external refrigerant system        
Items (a) and (b) require significant capital and downtime with a long delivery schedule besides modifications and/or additional driver and energy requirement of high pressure steam for the turbine drive. The option (a) could typically achieve about 20% additional capacity. The potential of capacity increase with option (b) is much more and also requires additional compression power, and increased capital and plot space than option (a). In most cases, these options are frequently not economically justifiable based on the payback criteria.
Suction chilling of Item (c) has been practiced for long time and is also an expensive option for process air compressors since it requires an external mechanical refrigeration system with additional compression energy and plot space. However, this option may be somewhat less expensive than the first two options (a and b) but provides only a modest increase in capacity and is rarely justified economically—evident from the fact that only a handful of plants implemented suction chilling in ammonia plants. However, it remains a common feature for gas turbines in power plants.
What is needed is a new approach which provides a significant increase in process air compression capability without extensive capital investment requirements in expensive external refrigeration systems, no additional power requirements for the air compressors, and no expensive modifications to the process air compressor.